Thank you for the feedback. I rebuilt emacs with the --with-x-toolkit=no
option and the problem disappeared.

Op za 24 sep. 2016 om 09:32 schreef Eli Zaretskii <eliz@gnu.org>:

> > From: Tijs Mallaerts <tijs.mallaerts@gmail.com>
> > Date: Fri, 23 Sep 2016 22:30:45 +0000
> >
> > To reproduce:
> > - keep the default theme (of emacs -q) or switch to another theme with a
> dark background (f.ex. deeper-blue)
> > - switch to the scratch buffer
> > - zoom in to +3
> > - hit Return a few times
> >
> > A slight difference of the background color is visible between the lines
> on which Return was hit and the blank
> > lines. The first column has a different background color as well.
>
> Not reproducible here, but my build is not a GTK+ build, and the OS is
> not kubuntu.
>
> Thanks.
>